The present invention relates to an organo polymeric material for liquid crystal orientation having a characteristic of increasing a pretilt angle and a liquid crystal display device employing the same as an alignment layer.
Generally, liquid crystal has both the fluidity of liquid and the optical properties of crystal and so is classified as a material having a mesophase between liquid and solid. Optical properties of the liquid crystal can be changed by either an electric field or heat. Liquid crystal display (LCD) devices using these properties of liquid crystal are one of the representative of flat panel display devices, along with plasma display and light emitting diode. FIG. 1 illustrates a cross-sectional view of a general LCD. In FIG. 1, reference numerals 1 and 1' represent polarizing plates, reference numerals 2 and 2' represent substrates, reference numerals 3 and 3' represent electrodes, reference numerals 4 and 4' represent insulating films, reference numerals 5 and 5' represent orientation layers, a reference numeral 6 represents a spacer and a reference numeral 7 represents a liquid crystal layer.
Physical properties of the liquid crystal are liable to change according to the molecular arrangement and so response characteristics thereof are greatly changed according to an external force such as an electric field. Also, since the pretilt angle of the liquid crystal is changed according to the material of the alignment layer and/or orientation treatment method, the concept of control of the pretilt angle of the liquid crystal could be understood by the control of the orientation of the liquid crystal molecules. FIG. 2 illustrates an azimuth relation of the liquid crystal molecules and the orientation surface, that is, the substrate. Here, the pretilt angle (.theta.) is an angle made by a substrate and a director of the liquid crystal, and azimuth angle of a liquid crystal director is represented as .psi..
Uniform alignment of the liquid crystal is not easily obtained by simply injecting the liquid crystal between upper and lower substrates. Therefore, generally, an orientation layer is formed on the substrate to obtain uniform orientation of the liquid crystal.
Alignment material which can control the pretilt angle of the liquid crystal as described above is essential in enhancing display characteristics of the LCD. In the case of a super twisted nematic LCD (STN-LCD), a large pretilt angle of the liquid crystal is needed to prevent the generation of a defect line, and while in the case of a thin film transistor LCD (TFT-LCD), orientating materials having a small pretilt angle and a large pretilt angle are used as an upper substrate or a lower substrate, respectively (and vice versa) to enhance the viewing angle (Journal of the SID 2/2, 31, 1994). For the case of a surface stabilized FLCD (SSFLCD) which is liable to generate orientation defects and has poor mechanical shock resestance, alignment materials having a large pretilt angle are used to change the structure of the smectic layer to compensate for the defects (Japan Display '92, 53 & 523). However, most of the commercially available conventional alignment materials have a small pretilt angle and the change of the pretilt angles thereof is greatly changed according to the change of conditions such as baking temperature and orientation condition during manufacture of the device.
Polyimide resin film has been widely used as an alignment material to orientate the liquid crystal molecules. The liquid crystal molecules are arranged along a rubbing direction and are orientated with a pretilt angle of 1.degree.-3.degree. by means of rubbed polyimide film formed on a substrate with a cloth in a predetermined direction. However, through this method of using rubbed organic resin film formed on the substrate, the pretilt angle of the liquid crystal molecules may only slightly be increased.
Another method of obtaining a large pretilt angle is an obliqud evaporation method of inorganic material such as silicon oxide without rubbing. However, the obliqud evaporation of inorganic material is complex and is not appropriate in practical production on an industrial scale, when compared with the rubbing method using organic resin film.
To solve these problems, the present inventors synthesized organic polymer containing fluorine, applied the material as an alignment material for an LCD and discovered after repeated research and development, that both uniform orientation and a large pretilt angle of the liquid crystal molecules can be obtained.